This invention relates to fuel cells and, in particular, to fuel cell plate structure used to establish fuel cell gas flow channels.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Molten carbonate fuel cells operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode.
In order to produce a useful power level, a number of individual fuel cells are stacked in series to form a fuel cell stack. Fuel cells in a molten carbonate fuel cell stack employ plate structure in establishing gas flow channels through the cells. The plate structure typically includes an electrically conductive separator plate, also called a bipolar separator plate, between adjacent cells. Particularly, the bipolar separator plate is used to separate an anode element of a first cell and a cathode element of a second cell, and to provide electrical contact with the current collectors in these cells.
A conventional bipolar separator plate includes a flat, rectangular, gas-impermeable plate member having a cathode surface facing one adjacent cell and an anode surface facing another adjacent cell. The bipolar plate typically also includes two pocket areas on each surface. These pocket areas may be formed by folding two opposite edges of the plate over the cathode surface and by folding the other two edges over the anode surface. An example of plate structure having such bipolar plate arrangement is disclosed in U.S. Pat. No. 6,372,374, assigned to the same assignee herein.
Anode and cathode current collectors are also part of the plate structure and abut the anode and cathode surfaces, respectively, and extend into the pocket areas of the bipolar separator. An anode and a cathode, in turn, abut the anode and cathode current collectors which along with the bipolar separator define anode and cathode gas flow channels for delivering fuel and oxidant gases to the respective electrodes.
An electrolyte matrix is disposed adjacent to the anode electrode and extends over the outer surfaces of the two pocket areas at the anode surface of the bipolar separator plate, while an electrolyte matrix is also disposed adjacent to the cathode electrode and extends over the outer surfaces of the two other pocket areas at the cathode surface of the separator plate. The plate structure having the bipolar separator plate, current collectors with matrices thus forms one-half of a first fuel cell and one-half of a second cell.
Stacking of these half-cell units results in complete fuel cells arranged in a stack. With such a fuel cell stack, the outer surfaces of the pocket areas of the bipolar separator plates act as rails to form wet seal areas with the electrolyte matrices. The pocket areas also are the inactive areas of the cells. The central areas of the bipolar separator plates formed by the cathode and anode surfaces between the pocket areas, in turn, are the active areas of the cells.
Forming the wet seal areas using the bipolar plate pocket areas is a practical and cost effective way to achieve gas tightness around the peripheral areas of the cells. However, a common problem associated with this arrangement is leaking of the fuel and oxidant gases flowing in the gas flow channels established by the plate structure from the active cell areas into the wet seal areas. Particularly, a portion of the fuel gas and a portion of the oxidant gas typically bypass the active cell areas by flowing into and through the wet seal areas without undergoing the desired electrochemical reaction. Moreover, in internally reforming fuel cells, where additional cooling is provided in the anode active areas through internal fuel reforming, the fuel bypassing the anode active areas is not reformed and therefore less cooling is provided in the cell active areas. As a result, the fuel cells may become overheated, and the efficiency and power output of the fuel cell stack are reduced. Accordingly, a means of inhibiting or retarding gas from flowing into the wet seal areas is needed to improve the fuel cell stack performance and efficiency.
Additionally, the wet seal areas are particularly vulnerable to electrochemical corrosion and oxidation. Therefore, the materials used in the wet seal areas need to be stable under electrochemical corrosion and oxidation conditions.
It is therefore an object of the present invention to overcome the above and other drawbacks of conventional fuel cell plate structure by adapting the plate structure to inhibit or retard gas from bypassing the active cell areas.
It is also an object of the present invention to provide a fuel cell plate structure further adapted to direct gas from the wet seal areas into the active cell areas.
It is a further object of the present invention to provide a fuel cell plate structure adapted as above set forth and using material in the wet seal areas which is stable in corrosive and oxidative environments.